The NAVSTAR Global Positioning System (GPS) is a Department of Defense satellite navigation system that uses a constellation of GPS navigation satellites in a space segment (SS) to transmit GPS signals and data from which a world wide User Segment (US) can derive accurate position, velocity and time.
A GPS Control Segment on the ground tracks the SS satellite constellation, and uplinks to each GPS satellite ephemeris data on its orbital characteristics and satellite clock correction parameters to precisely synchronize the on-board satellite atomic clock with reference to GPS system time. Each GPS satellite continually transmits a navigation signal that provides navigation message data--including time of transmission, satellite clock correction parameters and ephemeris data.
The navigation signal is transmitted over two carrier frequencies in the L band (L1 at 1575.42 and L2 at 1227.6 MHz), spread spectrum modulated With two pseudo random noise (PRN) codes: (a) a P-code (precision) with a seven day period (repeating its PRN sequence only once every seven days), providing for precision measurement of time, and (b) a C/A-code (clear/access or coarse/acquisition) with a one millisecond period, providing for rapid search and acquisition of the navigation signal from a given satellite (and hand-off to the more precise but harder to acquire P-code). Both L1 and L2 are modulated with the P-code (10.23 MHz), while only L1 carries the C/A-code (1.023 MHz), and each code modulation is further modulated with the 50 Hz navigation message data.
The problem to which the invention is directed is the design of an economic and high precision system of GPS measurements to permit accurate computation of pointing (azimuth and elevation) or attitude (roll, pitch/elevation and yaw/azimuth). More specifically, the design problem is to economically eliminate or control the distributed error sources that are typically encountered in GPS receivers, achieving precise performance necessary for accurate GPS pointing/attitude, in addition to the normal position, velocity and time computations. These error sources include satellite clock, satellite electrical path, receiver oscillator, code and carrier generators, receiver electrical path, receiver time bias error, satellite clock prediction, ephemeris radial prediction, ionospheric and other atmospheric signal paths.
For conventional GPS navigation (position and velocity in three dimensions), a User Segment GPS receiver tracks four GPS satellites, establishing synchronism with its navigation signal, and recovering the navigation message data. Ranges to the four satellites are determined by scaling the signal transit time by the speed of light, with the position of each satellite at the time of transmission being determined from the associated ephemeris parameters.
The need for a GPS receiver to include a precision clock (synchronized to GPS system time) is eliminated by the use of range measurements from four satellites. That is, the navigation problem for position is characterized by four unknowns--position in three dimensions and clock error (or fixed time bias)--requiring four user position equations to provide three estimated position coordinates and an estimate of the receiver time bias. In addition, velocity measurements are made by measuring the doppler shift in the carrier frequency of the navigation signal, with the error offset in the frequency of the receiver oscillator being compensated for by using four range rate (doppler) equations.
From each satellite, the GPS receiver is able to provide pseudo range and range rate (doppler) measurements that can be used for position and velocity computations. Pseudo range is the range measurement, with respect to the receiver clock, to any satellite being tracked based on signal transit time and satellite position before compensating for the fixed receiver time bias associated with any range measurement.
For GPS pointing applications, such as precision geodetic surveying, GPS interferometry techniques are used to define to a high degree of precision a baseline vector between a reference antenna (at a known location) and a second antenna (at an unknown location)--typically, this vector can be defined within a millimeter for relatively short baselines. The vector includes both the distance and the direction (azimuth and elevation) to the second antenna with respect to the reference antenna. Using this vector, the precise geodetic position of the unknown antenna can be computed by adding (in the same coordinate system) the measured vector to the known geodetic position of the reference antenna.
GPS interferometry uses differentially-processed carrier doppler phase measurements for the two locations to provide first order positioning of the second GPS receiver/antenna location--more precisely, the antenna phase center location--with respect to the reference GPS receiver/antenna location (References 1, 2 and 3, listed at the end of the Background). This technique can be extended to obtain attitude information using differential carrier doppler phase measurements for two GPS receivers/antennas at unknown locations arranged in a triangular pattern with respect to the reference antenna--the two measured vectors determine a plane characterized by roll, pitch and yaw.
In any GPS receiver design, tracking a GPS satellite requires synchronization with and demodulation of the carrier and PRN codes from the GPS navigation signals--a correlation process establishes carrier and code tracking loops that align selected GPS carrier and code (P or C/A) signals with corresponding replica carrier and code signals generated within the GPS receiver. In particular, the receiver measures apparent (pseudo range) transit time by measuring the phase shift between the GPS code signal and the receiver replica code signal--the receiver replica code is shifted until maximum correlation (within the error tolerance of the carrier and code tracking loops) is achieved between it and the received GPS code, with the time magnitude of the shift corresponding to measured pseudo range.
This tracking process of maintaining correlation between the P- or C/A-codes recovered from a selected incoming GPS navigation signal, and the corresponding receiver replica code, is a closed loop. For each GPS satellite being tracked, the selected GPS code/carrier signals are fed into the GPS inputs of the tracking channel's code and carrier correlators, while the replica inputs receive the receiver generated replica code and carrier. The resulting correlator outputs are split into in-phase (I) and quadrature (Q) signals, which are combined into code and carrier error signals. These error signals are fed into code and carrier tracking loop filters to generate corrections to the replica code and carrier generators, the corrected outputs of which are fed back to the code and carrier correlators as a closed loop.
The replica state for the tracking receiver changes as a function of two effects: (a) a time effect due to the signal state rate of change within the satellite as a function of time, and (b) a position effect due to the physical separation between the satellite and the receiver antenna. The physical separation between the satellite and the antenna is, in general, constantly changing due to the satellite orbital motion and, in the case of a dynamic user, due to the antenna motion, thereby producing a doppler effect on the code and carrier signals which is proportional to the net line of sight relative motion between the GPS antenna and the satellite antenna.
For a pointing application, if multiple GPS receiver/antennas are at fixed locations, or are rigidly attached to a single platform (such as on either end of a rigid beam), differential measurements can be made for the multiple antennas with respect to a single satellite. This differential measurement process eliminates the time effect (attributable to the satellite), so that only the relative position effect (attributable to the multiple antennas) remains. Measuring and processing relative position measurements yields the desired pointing vector (at least two antennas) or attitude vectors (at least three non-linear antennas).
This differential process for measuring relative position is commonly referred to as differential carrier doppler phase measurement. Differential carrier doppler phase measurement is not to be confused with range rate (doppler) measurements used for velocity computations--conventional range rate computations use fairly coarse doppler measurements that do not require precise measurement of the phase difference between the doppler shifted GPS signals arriving at different antennas. In contrast, to achieve significant pointing/ attitude accuracy, differential carrier doppler phase measurement must provide a highly precise measure of carrier phase difference with respect to the signals arriving at the different antennas.
Using separated GPS receivers for differential carrier doppler phase measurements inherently introduces differential receiver error sources that impact the precision of those measurements, and therefore, the pointing/attitude computations that use those measurements. These error sources include satellite clock errors, satellite electrical path errors, receiver clock errors, receiver electrical path errors, oscillator noise, code and carrier generator noise, time bias error in the range measurements, satellite clock prediction errors, ephemeris radial prediction errors and ionospheric errors. Several GPS processing techniques have been developed to eliminate the sources of receiver error caused by the introduction of separated GPS receivers (See references 4, 5, 6, listed at the end of the Background).
If analog multichannel GPS receivers are used, another significant error source is interchannel bias error--even if calibrated, interchannel bias changes as a function of both time and temperature. In current digital multichannel GPS receivers with precorrelation analog-to-digital conversion, the analog GPS signals from different satellites pass through a common analog RF/IF front-end, and are delayed by the same amount, producing a common interchannel bias because the analog circuit is common to all GPS signals. The differential digital signal delay paths can be precisely matched and are highly stable over time and temperature, so that the common interchannel bias is eliminated when the outputs attributable to different satellites (i.e., in different channels) are subtracted from each other in the pointing and attitude measurement process.
Even if these error sources can be eliminated or counteracted, three additional error sources significantly impact differential carrier doppler phase measurements used in pointing/attitude: (a) antenna differential phase center migration, (b) differential multipath, and (c) thermal noise.
An ideal GPS receiver antenna navigates the phase center of the GPS antenna when absolute positioning is performed because, ideally, all delay paths between the antenna phase center and the receiver correlators are equal and therefore cancel in the differential measurement process. However, in practice, the location of any GPS antenna phase center migrates as a function of the elevation and azimuth angle of each GPS satellite being tracked because of changes in the antenna gain and phase response at varying angles are not equal. These antenna phase center migration errors can be minimized by using phase matched antennas oriented in the same direction, such that any resulting errors cancel when the signals are differentiated.
Multipath errors arise because the direct-path GPS signal arrival is corrupted by associated multipath signals that arrive slightly later after reflecting from nearby reflecting surfaces. A significant source of multipath reflections is the location of reflecting surfaces near or below the horizon view of the GPS antenna. These multipath reflections can be minimized by antenna designs whose gain near and below the horizon is sufficiently low to reject multipath signals.
The precision of the differential carrier doppler phase measurements is ultimately limited by thermal noise, which is accurately predictable from the strength of the GPS signal, the noise figure of the receiver, and the bandwidth of the tracking loops. Thus the goal of GPS receiver design is to reduce the remaining sources of measurement error (including phase center migration and multipath) so as to be small compared with the thermal noise. In the case of the differential carrier doppler phase measurements used in pointing/attitude applications, an appropriate design goal is to make these error sources small compared to one degree of a carrier cycle at L-band--for the GPS L1 carrier at 1575.42 MHz, this corresponds to 0.5286 millimeters, and for the L2 carrier at 1227.6 MHz, this corresponds to 0.6784 millimeters.
Accordingly, a need exists for an economical GPS system that permits precise pointing or attitude measurements by eliminating or controlling distributed error sources.